[FEATURE] Configure custom environment variables

- Fix for #3097
- Could help plugins, like SEXTANTE, work better across platforms via abstracted coding based upon env vars
- In app prefs, configure variables and view current set, with tooltips to show pre-startup value (if any)
Add checkbox to filter non-QGIS-specific vars from current env vars table
Merge pull request #372 from dakcarto/iface_enviro_3

[FEATURE] Configure custom environment variables